Methods for producing ammonia and related systems

ABSTRACT

A method for producing ammonia comprises introducing a first feed stream to a positive electrode of an electrochemical cell. The electrochemical cell comprises the positive electrode, a negative electrode, and an electrolyte between the positive electrode and the negative electrode. A second feed stream comprising a nitrogen source is introduced to the negative electrode and a potential difference is applied between the positive electrode and the negative electrode to produce hydrogen ions, a first product stream comprising carbon monoxide, and a second product stream comprising ammonia. Additional methods and systems are disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 63/078,994, filed Sep. 16, 2020, the disclosure of which is hereby incorporated herein in its entirety by this reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract Number DE-AC07-05ID14517 awarded by the United States Department of Energy. The government has certain rights in the invention.

TECHNICAL FIELD

Embodiments of the disclosure relate to systems and methods directed to producing ammonia. The systems and associated methods are particularly directed towards so-called “green” processes (i.e., those that do not generate GHGs) for producing ammonia and related systems.

BACKGROUND

As the human population has continued to grow, the demand for fertilizers in the agriculture sector grows to meet demand. As a result, ammonia production (an important product in fertilizer) has also increased. Ammonia is synthesized through the Haber-Bosch (HB) process, wherein nitrogen reacts with hydrogen to produce ammonia, as follows:

N₂+3H₂→2NH₃  (1)

A source of hydrogen for the nitrogen reduction reaction come from hydrogen gas produced via (1) processing alkanes with steam (i.e., “steam reformation”) (eq. 2) or (2) a water shift reaction (eq. 3), as follows:

CH₄+H₂O→CO+3H₂  (2)

CO+H₂O→CO₂+H₂  (3)

The steam reformation/water shift reactions and ammonia synthesis occur in separate chambers in the conventional Haber Bosch process, and both reactions occur simultaneously. However, the water shift reaction generates CO₂, a known greenhouse gas (GHG) that must be captured and removed. The Haber Bosch process is responsible for 1.4% of the annual global CO₂ emissions and consumes between 1-3% of global energy production. Additionally, the steam reformation and water shift reactions require high temperatures and pressures to produce CO, CO₂, and H₂. Thus, synthesis of ammonia via the Haber Bosch process comes at the cost of increasing CO₂ production and large energy inputs, which can be expensive.

Some efforts have been made towards electrochemical methods for synthesizing ammonia to circumvent the need for systems that can tolerate high pressures and high temperatures to drive the oxidation and reduction reactions. Additionally, electrochemical methods for ammonia synthesis have been able to half the CO₂ emissions while consuming only 25% of global energy production under the conventional HB process. In such electrochemical cells, the anode catalyzes the oxidation of an alkane (such as methane) via steam reformation to produce hydrogen ions and electrons (eq. 4):

CH₄+2H₂O→CO₂+8H⁺+8e ⁻  (4)

The produced hydrogen ions travel through a proton conducting ceramic material to the cathode, where nitrogen gas reduction occurs to form ammonia (as in eq. 1).

However, such electrochemical methods produce CO₂ even though the reactions utilize less extreme conditions (i.e., by implementing different catalysts in the electrochemical cell). Accordingly, ammonia synthesis is not a truly green process, even though other GHGs such as CH₄ are consumed in the process.

BRIEF SUMMARY

A method for producing ammonia is disclosed and comprises introducing a first feed stream to a positive electrode of an electrochemical cell. The electrochemical cell comprises the positive electrode, a negative electrode, and an electrolyte between the positive electrode and the negative electrode. A second feed stream comprising a nitrogen source is introduced to the negative electrode and a potential difference is applied between the positive electrode and the negative electrode to produce hydrogen ions, a first product stream comprising carbon monoxide, and a second product stream comprising ammonia.

Another method for producing ammonia is disclosed. The method comprises introducing a first feed stream comprising methane and carbon dioxide to a positive electrode of an electrochemical cell. The positive electrode comprises an oxidation catalyst. The electrochemical cell comprises the positive electrode, a negative electrode, and an electrolyte between the positive electrode and the negative electrode. A second feed stream comprising nitrogen gas is introduced to the negative electrode. The negative electrode comprises a reduction catalyst. A potential difference is applied between the positive electrode and the negative electrode to oxidize the methane and carbon dioxide to produce carbon monoxide and to reduce the nitrogen gas to produce ammonia.

A system for producing ammonia is also disclosed and comprises an electrochemical cell, which comprises a positive electrode, a negative electrode, and an electrolyte between the positive electrode and the negative electrode. The positive electrode comprises an oxidation catalyst formulated to produce carbon monoxide. The negative electrode comprises a reduction catalyst formulated to produce ammonia.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a system in accordance with embodiments of the disclosure;

FIG. 2 is a flow chart of a process of producing ammonia in accordance with embodiments of the disclosure;

FIGS. 3A and 3B are SEM images of cross-sections of electrochemical cells in accordance with embodiments of the disclosure;

FIGS. 4A-4E are graphs showing properties of the electrochemical cell of FIG. 3A; and

FIG. 5A-5E are graphs showing properties of the electrochemical cell of FIG. 3B.

DETAILED DESCRIPTION

Disclosed herein are methods and systems for transforming greenhouse gases (GHG) into ammonia and other commercially desirable products. The methods and systems enable the production of industrially important products, such as ammonia and carbon monoxide, by utilizing known GHGs, such as carbon dioxide, without using water (e.g., steam). The methods and systems produce ammonia while simultaneously consuming carbon dioxide. The methods and systems provide a so-called “dry reformation process” instead of a steam reformation process as conventionally used in the Haber-Bosch process. The carbon dioxide (and other GHGs) are consumed from a feed stream by utilizing the carbon dioxide as a starting material without using steam as a starting material. The production of ammonia and carbon monoxide occurs at a low temperature, such as at a temperature of less than or equal to about 500° C.

The following description provides specific details, such as material compositions and processing conditions (e.g., temperatures, pressures, flow rates, etc.) in order to provide a thorough description of embodiments of the disclosure. However, a person of ordinary skill in the art will understand that the embodiments of the disclosure may be practiced without necessarily employing these specific details. Indeed, the embodiments of the disclosure may be practiced in conjunction with conventional systems and methods employed in the industry. In addition, only those process components and acts necessary to understand the embodiments of the disclosure are described in detail below. A person of ordinary skill in the art will understand that some process components (e.g., pipelines, line filters, valves, temperature detectors, flow detectors, pressure detectors, and the like) are inherently disclosed herein and that adding various conventional process components and acts would be in accord with the disclosure. In addition, the drawings accompanying the disclosure are for illustrative purposes only, and are not meant to be actual views of any particular material, device, or system.

As used herein, spatially relative terms, such as “beneath,” “below,” “lower,” “bottom,” “above,” “upper,” “top,” “front,” “rear,” “left,” “right,” and the like, may be used for ease of description to describe one element's or feature's relationship to another element(s) or feature(s) as illustrated in the figure. Unless otherwise specified, the spatially relative terms are intended to encompass different orientations of the materials in addition to the orientation depicted in the figure. For example, if materials in the figure are inverted, elements described as “below” or “beneath” or “under” or “on bottom of” other elements or features would then be oriented “above” or “on top of” the other elements or features. Thus, the term “below” can encompass both an orientation of above and below, depending on the context in which the term is used, which will be evident to one of ordinary skill in the art. The materials may be otherwise oriented (e.g., rotated 90 degrees, inverted, flipped) and the spatially relative descriptors used herein interpreted accordingly.

As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

As used herein, “and/or” includes any and all combinations of one or more of the associated listed items.

As used herein, the term “hydrocarbon” means and include a carbon-containing compound that includes at least one carbon (C1) atom.

As used herein, the term “configured” refers to a size, shape, material composition, material distribution, and arrangement of one or more of at least one system or apparatus facilitating operation of one or more of the structure and the system or apparatus in a pre-determined way.

As used herein, the term “substantially” in reference to a given parameter, property, or condition means and includes to a degree that one skilled in the art would understand that the given parameter, property, or condition is met with a small degree of variance, such as within acceptable manufacturing tolerances. For example, a parameter that is substantially met may be at least about 90% met, at least about 95% met, or even at least about 99% met.

As used herein, the terms “about” and “approximately” in reference to a numerical value for a particular parameter are inclusive of the numerical value and a degree of variance from the numerical value that one of ordinary skill in the art would understand is within acceptable tolerances for the particular parameter. For example, “about” in reference to a numerical value may include additional numerical values within a range of from 90.0 percent to 110.0 percent of the numerical value, such as within a range of from 95.0 percent to 105.0 percent of the numerical value, within a range of from 97.5 percent to 102.5 percent of the numerical value, within a range of from 99.0 percent to 101.0 percent of the numerical value, within a range of from 99.5 percent to 100.5 percent of the numerical value, or within a range of from 99.9 percent to 100.1 percent of the numerical value.

As used herein, the terms “comprising,” “including,” “containing,” “characterized by,” and grammatical equivalents thereof are inclusive or open-ended terms that do not exclude additional, unrecited elements or method acts, but also include the more restrictive terms “consisting of” and “consisting essentially of” and grammatical equivalents thereof.

As used herein, the term “may” with respect to a material, structure, feature or method act indicates that such is contemplated for use in implementation of embodiments of the disclosure and such term is used in preference to the more restrictive term “is” so as to avoid any implication that other, compatible materials, structures, features and methods usable in combination therewith should or must be excluded.

As used herein, the terms “catalyst material” and “catalyst” and their grammatical equivalents each mean and include a material formulated to promote one or more reactions, resulting in the formation of a product.

As used herein, the term “negative electrode” and grammatical equivalents means and includes an electrode having a relatively lower electrode potential in an electrochemical cell (e.g., lower than the electrode potential in a positive electrode therein).

Conversely, as used herein, the term “positive electrode” and grammatical equivalents means and includes an electrode having a relatively higher electrode potential in an electrochemical cell (e.g., higher than the electrode potential in a negative electrode therein).

As used herein, the term “electrolyte” and grammatical equivalents means and includes an ionic conductor, which can be in a solid state, a liquid state, or a gaseous state (e.g., plasma).

As illustrated in FIG. 1, a system 100 according to embodiments of the disclosure comprises an electrochemical cell 102, which comprises a positive electrode 104 (e.g., an anode), an electrolyte 106, and a negative electrode 108 (e.g., a cathode), where the electrolyte 106 is interposed between the positive electrode 104 and the negative electrode 108. The positive electrode 104 and the negative electrode 108 are electrically coupled to a power source 110. The system 100 comprises a first chamber 112 adjacent to the positive electrode 104, where the positive electrode 104 comprises an oxidation catalyst 114. The system 100 further comprises a second chamber 116 adjacent to the negative electrode 108, where the negative electrode 108 comprises a reduction catalyst 118. The first chamber 112 is coupled to a first inlet 120 that is configured to direct a first feed stream 122 into the first chamber 112. The second chamber 116 is coupled to a second inlet 124 that is configured to direct a second feed stream 126 into the second chamber 116. The first chamber 112 is coupled to a first outlet 128 that directs a first product stream 130 away from the first chamber 112. The second chamber 116 is coupled to a second outlet 132 that directs a second product stream 134 away from the second chamber 116. The first product stream 130 and the second product stream 134 including the desired products may be collected and recovered. The power source 110 is configured to apply a potential difference (e.g., voltage) between the negative electrode 108 and the positive electrode 104 of the electrochemical cell 102.

While not shown, the system 100 may include one or more apparatuses (e.g., heat exchangers, pumps, compressors, expanders, mass flow control devices, etc.) to adjust one or more of temperature, pressure, and flow rate of the first feed stream 122 and the second feed stream 126 delivered to the positive electrode 104 and to the negative electrode 108, respectively. The flow rates for the first feed stream 122 and second feed stream 126 may be adjusted depending on the compositions of the first feed stream 122 and second feed stream 126. The flow rates may range from about 20 ml min⁻¹ to about 150 ml min⁻¹. The temperature of the system 100 may be adjusted from about 25° C. to about 500° C.

The first feed stream 122 enters the system 100 through the first inlet 120 and passes into the first chamber 112 and over the positive electrode 104. The first feed stream 122 includes carbon dioxide and at least one hydrocarbon. The hydrocarbon may, for example, be an alkane, such as methane, ethane, propane, other linear alkane, or a combination thereof. The first feed stream 122 may, therefore, include carbon dioxide and the alkane. The first feed stream 122 may optionally include hydrogen gas as a source of hydrogen ions. By way of example only, the first feed stream 122 may include air, biogas, a carbon dioxide-containing feed stream from an industrial process, or a substantially pure source of carbon dioxide and the alkane. In certain embodiments the first feed stream 122 includes carbon dioxide and methane. In certain embodiments, the first feed stream 122 includes carbon dioxide, methane, and ethane. The hydrocarbon of the first feed stream 122 provides a source of hydrogen ions (e.g., protons) as a result of catalytic electrolysis, which are used to reduce (e.g., chemically reduce) the second feed stream 126 at the negative electrode 108 using the reduction catalyst 118. The carbon dioxide in the first feed stream 122 may be present in a liquid phase (e.g., CO₂ dissolved in an ionic liquid), a gaseous phase, or a combination thereof. The first feed stream 122 is substantially free of water, such as including less than about 5% water or less than about 1% water.

The positive electrode 104 may comprise an oxidation catalyst 114 that is formulated to oxidize starting materials within the first feed stream 122. In some embodiments, the oxidation catalyst 114 is on the surface of the positive electrode, and the oxidation catalyst is capable of interacting with starting materials in the first feed stream. The oxidation catalyst 114 may convert the starting materials within the first feed stream into a first product stream. The positive electrode 104 may be formed of and include at least one catalyst-doped material compatible with the material compositions of the electrolyte 106, the first feed stream 122, the negative electrode 108, and the operating conditions (e.g., temperature, pressure, current density, etc.) of the electrochemical cell 102.

The oxidation catalyst 114 may comprise a single material (e.g., a single metal) or at least two materials. For example, the oxidation catalyst 114 may be a supported metal catalyst doped with a metal. The metal may be palladium (Pd), platinum (Pt), rhodium (Rh), ruthenium (Ru), iridium (Ir), nickel (Ni), cobalt (Co), or a combination thereof. The supported metal catalyst may be a perovskite material, such as BZCYYb, BSNYYb, doped BaCeO₃, BaZrO₃, Ba₂(YSn)O_(5.5), Ba₃(CaNb₂)O₉. The perovskite material of the oxidation catalyst 114 may be doped with Ni, Au, Pd, Pt, Ir, Rh, Ru, Co, or a combination thereof. The oxidation catalyst 114 may be BZCYYb doped with Ni (or Ni—BZCYYb). The metal support may be PrBaMn₂O_(5+δ) (PBM) material doped with a metal from the group as described above. In some embodiments, the positive electrode 104 is PrBaMn2O5+δ (PBM) doped with Pt (or PBM/Pt) as the oxidation catalyst 114. The oxidation catalyst 114 may also be a metal-doped perovskite, where the perovskite may be LaAl_(0.2)Ni_(0.8)O₃ doped with iridium (Ir), palladium (Pd), platinum (Pt), rhodium (Rh), ruthenium (Ru), or a combination thereof. In some embodiments, the positive electrode 104 comprises Ni—BZCYYb, where surface Ni species are the oxidation catalyst 114. In additional embodiments, the positive electrode 104 comprises NiAu—BZCYYb, where the surface NiAu species are the oxidation catalyst 114.

In some embodiments, the oxidation catalyst 114 of the positive electrode 104 converts methane (CH₄) and carbon dioxide (CO₂) of the first feed stream 122 to carbon monoxide (CO) according to the following reaction:

CH₄+CO₂→2CO+4H⁺+4e ⁻  (5)

In some embodiments, the first feed stream 122 includes ethane (C₂H₆). When ethane is present, the positive electrode 104 catalyzes the following reaction:

C₂H₆→C₂H₄+2e ⁻+2H⁺  (6)

The generated hydrogen ions are transported from the positive electrode 104 through the electrolyte 106 to the negative electrode 108. The hydrogen ions may be involved in the hydrogenation (a reduction reaction) of starting materials in the second feed stream 126, where the second feed stream 126 is introduced into a second chamber 116 via second inlet 124 and passed through the negative electrode 108. The second feed stream 126 may comprise a nitrogen source, such as nitrogen gas (N₂), nitrous oxide (N₂O), nitrogen monoxide (NO), or a combination thereof. The second feed stream 126 optionally comprises water. In some embodiments, the second feed stream 126 comprises one or more of dinitrogen, nitrogen monoxide, nitrogen dioxide, or a combination thereof and water. The second feed stream 126 may, for example, include air, a nitrogen source-containing feed stream from an industrial process, or a substantially pure nitrogen source. The hydrogen ions produced from the carbon dioxide in the first feed stream 122 may be reacted with the nitrogen source of the second feed stream 126 to produce the ammonia.

The reduction catalyst 118 of the negative electrode 108 is formed of and includes a material that interacts with and reduces (e.g., hydrogenates) the one or more components of the second feed stream 126. When the second feed stream 126 comprises, for example, dinitrogen (N₂), the reduction catalyst 118 of the negative electrode 108 catalyzes the following reactions:

N₂+6e ⁻+6H⁺→2NH₃  (7)

2H⁺+2e ⁻→H₂  (8)

N₂+3H₂→2NH₃  (9)

The reduction reactions of dinitrogen gas and hydrogen ions to form ammonia and hydrogen gas, respectively, compete with each other at the negative electrode 108. In some embodiments the negative electrode 108 selectively reduces dinitrogen, as illustrated in eq. 7, over reducing hydrogen ions that diffuse to the negative electrode 108, as illustrated in eq. 8. In some embodiments, the negative electrode 108 chemically reduces dinitrogen using hydrogen formed in eq. 8, as illustrated in eq. 9. The reduced nitrogen gas may be ammonia or hydrazine (for partial reduction).

The negative electrode 108 may comprise a single material (e.g., a single metal) or at least two materials (e.g., a bimetallic material). Similar to the positive electrode 104, the negative electrode 108 may be formed of and include at least one catalyst-doped material compatible with the material compositions of the electrolyte 106, the second product stream 134, the positive electrode 104, and the operating conditions of the electrochemical cell 102. The negative electrode 108 may be an alloy, such as an AgPd alloy or a Ni-based alloy. The negative electrode 108 may, for example, comprise a cermet material comprising at least one catalyst material including one or more of a metal, metal alloy, or at least one perovskite, such as a doped perovskite cermet material, denoted as M-perovskite, where M may be a metal, such as a noble metal (Pt, Pd, Rh, Ru, Ir), Fe, Ag, or a combination thereof, or another material. For example, and not by limitation, the negative electrode 108 may comprise M-BZCYYb, M-BSNYYv, M-PrBa_(0.5)Sr_(0.5)Co_(1.5)Fe_(0.5)O_(5+δ) (M-PBSCF), M-PrNi_(0.5)Co_(0.5)O_(3−δ) (M-PNC), M-Pr_(0.5)Ba_(0.5)Co_(x)Fe_(1−x) ^(O) ₃, M-BaCe_(0.9)Y_(0.1)O₃, M-Pr_(0.5)Ba_(0.5)FeO₃, M-BaCeO₃, M-BaZrO₃, M-Ba₂(YSn)O_(5.5), M-Ba₃(CaNb₂)O₉), an MNi/perovskite (such as RuNi/perovskite) cermet (MNi-perovskite, such as RuNi-perovskite) material (e.g., MNi-BZCYYb, MNi-BSNYYb, MNi-PBSCF, MNi-PNC, MNi-Pr_(0.5)Ba_(0.5)Co_(x)Fe_(1−x)O₃, MNi-Pr_(0.5)Ba_(0.5)FeO₃, MNi-BaCeO₃, MNi-BaZrO₃, MNi-Ba₂(YSn)O_(5.5), MNi-Ba₃(CaNb₂)O₉), an MCe/perovskite cermet (such as a RuCe-perovskite) material (e.g., MCe-BZCYYb, MCe-BSNYYb, MCe-PBSCF, MCe-PNC, MCe-Pr_(0.5)Ba_(0.5)Co_(x)Fe_(1−x)O₃, MCe-Pr_(0.5)Ba_(0.5)FeO₃, MCe-BaCeO₃, MCe-BaZrO₃, MCe-Ba₂(YSn)O_(5.5), MCe-Ba₃(CaNb₂)O₉), and an MNiCe/perovskite cermet (RuNiCe-perovskite) material (e.g., RuNiCe—BZCYYb, RuNiCe—BSNYYb, RuNiCe—PBSCF, RuNiCe—PNC, RuNiCe—Pr_(0.5)Ba_(0.5)Co_(x)Fe_(1−x)O₃, RuNiCe—Pr_(0.5)Ba_(0.5)FeO₃, RuNiCe—BaCeO₃, RuNiCe—BaZrO₃, RuNiCe—Ba₂(YSn)O_(5.5), RuNiCe—Ba₃(CaNb₂)O₉. The negative electrode 108 may be, for example, a metal-doped ceria, such as lanthanum-doped ceria (LDC), samarium-doped ceria (SDC), or a combination thereof.

The reduction catalyst 118 of the negative electrode 108 may comprise a dopant integrated into the at least one catalyst material. For example, the dopant of the reduction catalyst 118 may comprise a metal-doped ceria, such as lanthanum-doped ceria (LDC) or samarium-doped ceria (SDC). The metal-doped ceria may be further doped with a metal, such as a noble metal (e.g., Pt, Pd, Rh, Ir, Ru) or iron (Fe). In some embodiments, the negative electrode 108 is PBSCF and comprises a reduction catalyst 118 that is Ru-doped LDC (Ru/LDC). The reduction catalyst 118 may further comprise a metal hydroxide species. In some embodiments, the reduction catalyst 118 is Ru-doped LDC having a Ru—OH surface species. In other embodiments, the reduction catalyst may be a potassium- or aluminum-modified Fe—BaCe_(0.9)Y_(0.1)O₃. In some embodiments, the reduction catalyst 118 comprises single atoms (e.g., single Ru atoms), nanoclusters (e.g., Ru—NCs), or nanoparticles (e.g., Ru-NPs).

The positive electrode 104 and the negative electrode 108 may individually exhibit any desired dimensions (e.g., length, width, thickness) and any desired shape (e.g., a cubic shape, cuboidal shape, a tubular shape, a tubular spiral shape, a spherical shape, a semi-spherical shape, a cylindrical shape, a semi-cylindrical shape, a conical shape, a triangular prismatic shape, a truncated version of one or more of the foregoing, and irregular shape) as are known in the art. For example, the dimensions and the shapes of the positive electrode 104 and the negative electrode 108 may be selected relative to the dimensions and the shape of the electrolyte 106 such that the electrolyte 106 substantially intervenes between opposing surfaces of the positive electrode 104 and the negative electrode 108. The material compositions of the positive electrode 104 and the negative electrode 108 may be selected relative to one another, relative to the electrolyte 106, the components of the first feed stream 122, and/or the components of the second feed stream 126. The oxidation catalyst 114 and the reduction catalyst 118 may be incorporated into the positive electrode 104 and the negative electrode 108 by conventional techniques.

The electrolyte 106 comprises at least one electrolyte material exhibiting ionic conductivity, such as H⁺ conductivity. In some embodiments, the electrolyte 106 is a proton exchange membrane (PEM). The electrolyte 106 enables H⁺ to move from the positive electrode 104 to the negative electrode 108. The thickness of the electrolyte 106 should comprise a thickness to effectively transfer H⁺ but not so thick as to require a large over potential to sustain H⁺ transport. For instance, when the thickness of the positive electrode 104 and the negative electrode 108 may be substantially the same, the thickness of the electrolyte 106 is at least the sum of the thicknesses of the positive electrode 104 and the negative electrode 108. In some embodiments, the thickness of the electrolyte 106 is substantially the same as one of the positive electrode 104 or the negative electrode 108. In some embodiments, the positive electrode 104 is of sufficient thickness to provide mechanical support for the system 100, and the thickness of the electrolyte 106 is about 10 μm.

The electrolyte 106 may be formed of a material that exhibits an ionic conductivity of greater than or equal to about 10⁻² S/cm (e.g., within a range of from about 10⁻² S/cm to about 1 S/cm) at one or more temperatures within a range of from about 150° C. to about 650° C. (e.g., from about 300° C. to about 500° C.). In addition, the electrolyte 106 may be formulated to remain substantially adhered (e.g., laminated) to the positive electrode 104 and the negative electrode 108 at relatively high current densities, such as at current densities greater than or equal to about 0.1 amperes per square centimeter (A/cm²) (e.g., greater than or equal to about 0.5 A/cm², greater than or equal to about 1.0 A/cm², greater than or equal to about 2.0 A/cm², etc.). For example, the electrolyte 106 may comprise one or more of a solid acid material, a polybenzimidazole (PBI) material (e.g., a doped PBI material), and a BZCYYb material (e.g., BaZr_(0.1)Ce_(0.7)Y_(0.1)Yb_(0.1)O_(3−δ) or a doped variant). The material composition of the electrolyte 106 may provide the electrolyte 106 with enhanced ionic conductivity at a temperature within the range of from about 150° C. to about 650° C. as compared to conventional electrolytes (e.g., membranes employing conventional electrolyte materials, such as yttria-stabilized zirconia (YSZ)) of conventional electrochemical cells.

The electrolyte 106 comprises a perovskite material (e.g., a BZCYYb, a BSNYYb, a doped BaCeO₃, a doped BaZrO₃.Ba₂(YSn)O_(5.5), Ba₃(CaNb₂)O₉, etc.) having an operational temperature within a range of from about 350° C. to about 650° C., the negative electrode 108 may comprise a catalyst-doped perovskite material compatible with the perovskite material of the electrolyte 106.

In some embodiments, the electrolyte 106 is formed of and includes at least one perovskite material having an operational temperature (e.g., a temperature at which the H⁺ conductivity of the perovskite material is greater than or equal to about 10⁻² S/cm, such as within a range of from about 10⁻² S/cm to about 10⁻¹ S/cm) within a range of from about 350° C. to about 650° C. In some embodiments, the perovskite material is a proton conducting ceramic. As a non-limiting example, the electrolyte 106 may comprise one or more of a yttrium- and ytterbium-doped barium-zirconate-cerate (BZCYYb), a yttrium- and ytterbium-doped barium-strontium-niobate (BSNYYb), doped barium-cerate (BaCeO₃) (e.g., yttrium-doped BaCeO₃ (BCY)), doped barium-zirconate (BaZrO₃) (e.g., yttrium-doped BaCeO₃ (BZY)), barium-yttrium-stannate (Ba₂(YSn)O_(5.5)); and barium-calcium-niobate (Ba₃(CaNb₂)O₉). In some embodiments, the electrolyte 106 comprises BZCYYb (e.g., BaZr_(0.1)Ce_(0.7)Y_(0.1)Yb_(0.1)O_(3−δ) and BaZr_(0.4)Ce_(0.4)Y_(0.1)Yb_(0.1)O_(3−δ)).

The system 100 may be equipped with heating apparatuses that maintain substantially the same temperature across the system 100. The temperature within the system 100 may be maintained at about 500° C. or less. In some embodiments, the system 100 operates at a temperature of about 400° C. or less.

While FIG. 1 illustrates the system 100 as including a single electrochemical cell 102, the system 100 may be used in series or in parallel with other systems to maximize production (i.e., scale-up) of ammonia simultaneous with consumption of carbon dioxide.

Embodiments of the disclosure will now be described with reference to FIG. 2, which is a flow chart illustrating a method 200 for producing ammonia. The method 200 is used to convert carbon dioxide into ammonia and other commercially desirable products, such as carbon monoxide. The method 200 produces ammonia while simultaneously consuming carbon dioxide. As shown in act 202 the process comprises introducing (e.g., providing) the first feed stream 122 to the positive electrode 104 of the electrochemical cell 102 (see FIG. 1), which includes the positive electrode 104, the negative electrode 108, and the electrolyte 106 as described above. The method 200 may include introducing (e.g., providing) the second feed stream 126 to the negative electrode 108, as shown in act 204. As shown in act 206, a potential difference is applied between the positive electrode 104 and the negative electrode 108 of the electrochemical cell 102 to produce hydrogen ions, and the hydrogen ions diffuse through the electrochemical cell 102. Applying the potential difference between the positive electrode 104 and the negative electrode 108 produces the first product stream 130, where the first product stream 130 comprises carbon monoxide. Additionally, applying the potential difference between the positive electrode 104 and the negative electrode 108 produces the second product stream 134, where the second product stream 134 comprises ammonia. The reactions within the system 100 may occur at a temperature of about 500° C. In some other embodiments, the act 206 occurs at a temperature of about 400° C.

As described above, the first feed stream 122 comprises carbon dioxide. In some embodiments, the first feed stream 122 comprises carbon dioxide and one or more alkane. In some embodiments, the first feed stream 122 comprises carbon dioxide and methane. In other embodiments, the first feed stream 122 comprises carbon dioxide, methane, and another alkane, such as ethane. In some embodiments, the first feed stream 122 comprises carbon dioxide and one or more alkane and is substantially free of water.

As described above, the second feed stream 126 comprises dinitrogen, nitrogen monoxide, nitrogen dioxide, or a combination thereof. In some embodiments, the second feed stream 126 comprises one or more of dinitrogen, nitrogen monoxide, nitrogen dioxide, and water. In other embodiments, the act 204 of introducing the second feed stream 126 over the negative electrode 108 comprises passing the second feed stream 126 over a Ru-LDC catalyst over a PBSCF material. In yet other embodiments, passing the second feed stream 126 over a Ru-LDC reduction catalyst 118 over a PBSCF material comprises interacting the second feed stream 126 with a metal hydroxide catalyst, such as Ru—OH/LDC.

As described above, applying the potential difference between the positive electrode 104 and the negative electrode 108 of the electrochemical cell produces the first product stream 130 comprising carbon monoxide over the positive electrode 104. In some embodiments, producing the first product stream 130 comprising carbon monoxide further comprises forming the first product stream 130 that is substantially free of carbon dioxide. If the first feed stream 122 comprises carbon dioxide and at least one hydrocarbon, then the first product stream 130 is substantially free of carbon dioxide and the at least one hydrocarbon, such as containing less than about 10% of carbon dioxide and the at least one hydrocarbon. In some embodiments, the first product stream 130 may comprise less than about 5% of carbon dioxide and the at least one hydrocarbon.

As described above, applying a potential difference between the positive electrode 104 and the negative electrode 108 of the electrochemical cell produces the second product stream 134. The applied potential difference produces hydrogen ions that diffuse through the cell and to the negative electrode 108. At the negative electrode 108, the hydrogen ions react with the nitrogen source of the second feed stream 126 to produce ammonia in the second product stream 134. In other words, the second product stream 134 may comprise ammonia, as produced from the reduction of dinitrogen, nitrogen monoxide, nitrogen dioxide, or a combination thereof. In some embodiments, the second product stream 134 may also comprise hydrazine (the partially hydrogenated product). In some embodiments, applying a potential difference between the positive electrode 104 and the negative electrode 108 simultaneously produces carbon monoxide and ammonia.

The first product stream 130 and the second product stream 134 including the desired products may be collected and recovered. The first product stream 130 contains CO, which may be used as a starting material (e.g., a feed stock) to produce a commodity product. The commodity product may include, but is not limited to, acetic acid, formic acid, formaldehyde, methanol, a formate, a methylated amine, an alcohol other than methanol, a carboxylic acid, a formamide, an aldehyde, a polycarbonate or other commercially valuable compound or product. Similarly, the ammonia of the second product stream 134 may be used as fertilizer, a fuel, or other commercially valuable compound or product.

The system 100 containing the electrochemical cell 102 and the methods according to embodiments of the disclosure are advantageous over conventional systems for synthesizing ammonia. Under the conventional HB configurations and electrochemical methods, carbon dioxide is necessarily produced as a product of steam reformation. In contrast, in the methods according to embodiments of the disclosure, the carbon dioxide is converted into carbon monoxide, while simultaneously producing ammonia. Therefore, carbon dioxide is consumed rather than produced. The methods according to embodiments of the disclosure also do not utilize water (as steam) to drive oxidation. Concomitant oxidation of the alkane over the positive electrode provides hydrogen ions and electrons for ammonia synthesis over the negative electrode. Additionally, the method according to embodiments of the disclosure may be advantageous over conventional methods because the system may be operated at intermediate temperatures, such as from about 300° C. to about 500° C. Therefore, the system 100 and method 200 may be utilized in on-site CO₂ conversion and ammonia synthesis within a single system. Additionally, because CO₂ is consumed in the process of ammonia synthesis, a carbon dioxide capture act is not needed.

The following examples serve to explain embodiments of the disclosure in more detail. These examples are not to be construed as being exhaustive or exclusive as to the scope of this disclosure.

Example 1

An electrochemical cell, such as the electrochemical cell 102 in FIG. 1, was formed and used to convert nitrogen gas to ammonia. The electrochemical cell 300A shown in FIG. 3A included a Ni—BZCYYb anode, a BZCYYb electrolyte, and a doped PrBa_(0.5)Sr_(0.5)CoFeO_(5+δ) (PBSCF) cathode. The electrochemical cell 300B shown in FIG. 3B included a PrBaMn₂O_(5+δ)+Pt (PBM/Pt) anode, a doped PBSCF cathode, and a BZ₄CYYb—S electrode.

The electrochemical cell 300A was tested for conversion of nitrogen gas to ammonia using three differently doped cathodes: (1) ruthenium doped PBSCF (P/Ru), (2) ruthenium doped PBSCF integrated into a lanthanum doped ceria (LDC) lattice (P/LDCRu-D), and (3) another ruthenium doped PBSCF integrated into a lanthanum doped ceria (LDC) lattice (P/LDCRu-W). P/LDCRu-D was produced by exposing the cathode material to dry air (i.e., substantially free of moisture). P/LDCRu-W was produced by exposing the cathode material to ambient air, which was presumed to contain water.

4A-4E are graphs illustrating properties of the electrochemical cells 300A (FIG. 3A), where FIG. 4A illustrates total current generated from the electrochemical cells. FIG. 4B is a graph illustrating a comparison of the faradaic efficiencies (FEs) of the electrochemical cells. FIG. 4C is a graph illustrating a comparison of current generated by the electrochemical cells. FIG. 4D is a bar graph illustrating a comparison of the production rate (PR) of the electrochemical cells. FIG. 4E is a graph illustrating the current density (solid line) and faradaic efficiency (empty pentagons) of the electrochemical cell over time.

Total current densities of the electrochemical cell 300A with P/Ru, P/LDCRu-D, and P/LDCRu-W cathodes for the conversion of N₂ into NH₃ are shown in FIG. 4A and were measured by conventional techniques. H₂ gas was used as the proton source over the anode and flowed at a rate of 60 mL min⁻¹. At the cathode, N₂ was flowed at a rate of 20 mL min⁻¹. The use of P/Ru as the cathode did not produce an appreciable amount of ammonia, as illustrated by a lower total current density (J_(total)). The use of the P/LDCRu-D and P/LDCRu-W cathodes produced more NH₃ than the P/Ru catalyst. The faradaic efficiency (FE) of the electrochemical cell 300A using the different cathode catalysts (P/Ru, P/LDCRu-D, and P/LDCRu-W) are illustrated in FIGS. 4B and 4C and were measured by conventional techniques. The FE of the different cathode catalysts of the electrochemical cells varied as a function of the potential bias (E_(bias)), with the P/LDCRu-W catalyst showing the best FE of 23.1% for ammonia production at a E_(bias) of ˜0.6V from a N₂ stream over the cathode and a H₂ stream over the anode. The temperature of the chamber was about 400° C. Without being bound by any theory, the active catalytic species of the P/LDCRu-W electrode was proposed to be a surface Ru—OH species.

The production rate (PR) was determined for each of the different cathode catalysts, as illustrated in FIG. 4D. The cathode catalysts were able to convert N₂ using hydrogen ions generated from H₂ at the anode to produce ammonia at the cathode. The PR was found to change as a function of E_(bias). The highest observed PR was measured to be 2.191 mol h⁻¹ m⁻².

The stability of the electrochemical cell having a P/LDCRu-W cathode, Ni—BSCYYb anode, and BZCYYb electrolyte was tested over the course of 550 hours while measuring the total current generated (solid lines) and the FE for ammonia production (dots) over that time frame, as illustrated in FIG. 4E. The E_(bias) was set to 0.6V and kept constant over the time frame. The temperature was set to 400° C. and also kept constant over the time frame.

Example 2

An electrochemical cell having a PBM/Pt anode, a P/LDCRu-W cathode, and a BZCYYb—S electrolyte was tested for ammonia production, as illustrated in FIGS. 5A through 5E. FIG. 5A-5E are graphs illustrating properties of the electrochemical cell (FIG. 3B), where FIG. 5A is a graph illustrating a comparison of current generated by the electrochemical cells. FIG. 5B is a graph illustrating the faradaic efficiency (FE) and production rate (PR) of the electrochemical cells. FIG. 5C is a graph illustrating a comparison of current generated by the electrochemical cells. FIG. 5D is a graph illustrating the faradaic efficiency and production rate of the electrochemical cells. FIG. 5E is a graph illustrating the current density (solid line) and faradaic efficiency (empty pentagons) of the electrochemical cell over time.

A feed stream of a 1:1 CO₂:CH₄ mixture was introduced over the anode at a flow rate of about 60 mL min⁻¹, and a feed stream of N₂ gas was introduced over the cathode. The total current density (dark squares) of the electrochemical cell and the current density generated attributable to the formation of ammonia (light circles) are shown in FIG. 5A.

The faradaic efficiency (FE, dark squares) and the production rate (PR, light circles) of the electrochemical cell are illustrated in FIG. 5B. As the potential bias increased, the FE of the electrochemical cell reached a maximum around 0.6 V and slowly decreased. As the potential bias increased, the PR of the electrochemical cell increased.

When using C₂H₆ in the feed stream to the anode and N₂ in the feed stream to the cathode, the total current density of the electrochemical cell and the current generated due to ammonia increased as a function of increasing E_(bias), as illustrated in FIG. 5C. The FE and PR for ammonia production of the electrochemical cell under these conditions is illustrated in FIG. 5D. The FE reached a maximum around 0.6V, while the PR of the cell continued to increase as a function of increasing E_(bias).

The stability of the electrochemical cell over 30 hours, while switching the potential bias and feed stream contents over the anode, is shown in FIG. 5E. N₂ gas was flowed over the cathode. The total current generated throughout the time frame is shown in solid lines, and the FE for NH₃ production is shown in empty pentagons. Over the time frame, switching the potential bias and feed stream (i.e., the proton source) among H₂, C₂H₆, and CO₂:CH₄ did not appear to appreciably decrease the total current generated in the electrochemical cell. These conditions did not appear to change the FE for NH₃ production over the same time frame, indicating that the electrochemical cell was stable and capable of tandem dry reforming (oxidation of a hydrogen source and carbon dioxide) and ammonia production.

While the disclosure is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, the disclosure is not limited to the particular forms disclosed. Rather, the disclosure is to cover all modifications, equivalents, and alternatives falling within the scope of the following appended claims and their legal equivalent. For example, elements and features disclosed in relation to one embodiment may be combined with elements and features disclosed in relation to other embodiments of the disclosure. 

What is claimed is:
 1. A method for producing ammonia comprising: introducing a first feed stream comprising carbon dioxide to a positive electrode of an electrochemical cell, the electrochemical cell comprising the positive electrode, an electrolyte, and a negative electrode; introducing a second feed stream comprising a nitrogen source to the negative electrode; and applying a potential difference between the positive electrode and the negative electrode of the electrochemical cell to produce hydrogen ions, a first product stream comprising carbon monoxide, and a second product stream comprising ammonia.
 2. The method of claim 1, wherein introducing a first feed stream to a positive electrode comprises introducing the first feed stream comprising the carbon dioxide and one or more alkane.
 3. The method of claim 2, wherein introducing a first feed stream to a positive electrode comprises introducing the first feed stream comprising carbon dioxide, methane, and another alkane.
 4. The method of claim 2, wherein applying a potential difference between the positive electrode and the negative electrode comprises producing the first product stream substantially free of carbon dioxide and the one or more alkane.
 5. The method of claim 1, wherein introducing a first feed stream comprising carbon dioxide to a positive electrode of an electrochemical cell comprises introducing the first feed stream substantially free of water.
 6. The method of claim 1, wherein introducing a first feed stream comprising carbon dioxide to a positive electrode comprises introducing the first feed stream to the positive electrode comprising an oxidation catalyst, the oxidation catalyst comprising a perovskite doped with a metal of palladium (Pd), platinum (Pt), iridium (Ir), ruthenium (Ru), rhodium (Rh), nickel (Ni), cobalt (Co), or a combination thereof.
 7. The method of claim 1, wherein introducing a second feed stream comprising a nitrogen source to the negative electrode comprises introducing the second feed stream to the negative electrode comprising a reduction catalyst, the reduction catalyst comprising a metal hydroxide catalyst.
 8. The method of claim 1, wherein introducing a second feed stream comprising a nitrogen source to the negative electrode comprises reducing the gas at a temperature of about 500° C. or less.
 9. The method of claim 1, wherein introducing a second feed stream comprising a nitrogen source to the negative electrode comprises introducing the second feed stream comprising nitrogen, nitrogen monoxide, nitrogen dioxide, or a combination thereof.
 10. The method of claim 1, wherein applying a potential difference between the positive electrode and the negative electrode of the electrochemical cell to produce a second product stream comprises diffusing the hydrogen ions through the electrolyte and reacting the hydrogen ions with the nitrogen source to produce the ammonia.
 11. The method of claim 1, wherein applying a potential difference between the positive electrode and the negative electrode of the electrochemical cell to produce hydrogen ions, a first product stream comprising carbon monoxide, and a second feed stream comprising ammonia comprises simultaneously producing the carbon monoxide and the ammonia.
 12. The method of claim 1, further comprising introducing a stream comprising water to the negative electrode.
 13. A method for producing ammonia comprising: introducing a first feed stream comprising methane and carbon dioxide to a positive electrode of an electrochemical cell, the electrochemical cell comprising the positive electrode, a negative electrode, and an electrolyte between the positive electrode and the negative electrode and the positive electrode comprising an oxidation catalyst; introducing a second feed stream comprising nitrogen gas to the negative electrode, the negative electrode comprising a reduction catalyst; and applying a potential difference between the positive electrode and the negative electrode to oxidize the methane and carbon dioxide to produce carbon monoxide and to reduce the nitrogen gas to produce ammonia.
 14. The method of claim 13, wherein introducing a second feed stream to the negative electrode comprises interacting the second feed stream with the reduction catalyst comprising a metal hydroxide integrated into a doped perovskite or supported metal catalyst.
 15. The method of claim 14, wherein introducing a second feed stream to the negative electrode comprises interacting the second feed stream with the reduction catalyst comprising a comprising a metal hydroxide comprising a metal selected from the group of rhodium (Rh), ruthenium (Ru), palladium (Pd), platinum (Pt), iridium (Ir), nickel (Ni), cobalt (Co), or a combination thereof.
 16. The method of claim 15, wherein introducing a second feed stream to the negative electrode comprises introducing the second feed stream comprising water and one or more of dinitrogen, nitrogen monoxide, nitrogen dioxide, or a combination thereof.
 17. A system for producing ammonia comprising: an electrochemical cell comprising: a positive electrode comprising an oxidation catalyst formulated to produce carbon monoxide; a negative electrode comprising a reduction catalyst formulated to produce ammonia; and an electrolyte between the positive electrode and the negative electrode.
 18. The system of claim 17, wherein the positive electrode comprises a supported metal catalyst.
 19. The system of claim 17, wherein the negative electrode comprises a reduction catalyst comprising a lanthanum-doped ceria doped with platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), or iron (Fe).
 20. The system of claim 17, wherein the negative electrode comprises a metal hydroxide catalyst. 